Eelco Franz (RIVM)

Summary of presentation:
Since mid-2023, the Netherlands has faced a marked and sustained rise in Salmonella enterica serovar Enteritidis (SE) infections in humans, paralleled by an increase in SE-positive laying hen flocks. This rising trend in human infections reverses the long-standing decline in SE incidence following EU-harmonized control programs, posing a growing public health concern. National laboratory surveillance data showed that non–travel-associated human SE cases rose from a pre-COVID annual average of 281 (2017–2019) to 427 in 2023, which was primarily driven by an outbreak linked to the inadequate processing of SE positive eggshells before their use in poultry feed. Despite successful intervention in this outbreak, case numbers remained elevated and increasing, with 401 cases in 2024 and 465 cases reported from January through October 2025. This increase coincides with a rise in SE-positive laying-hen flocks, from an average of 30 annually during 2018–2022 to 74 in 2023, 81 in 2024, and 50 in the first half of 2025. A significant temporal correlation was observed between SE-positive flocks and human SE cases. The diffuse pattern of numerous small clusters based on whole genome sequencing (WGS) suggests widespread sectoral transmission rather than point-source outbreaks, requiring an expansion of only farm-specific interventions to industry-wide control strategies. In addition, preliminary analysis show that strains are often specific to farms and are found in subsequent flocks of the same farm, which calls for more intense control and prevention measures at farm level. Implemented interventions by the laying-hen sector include increased flock testing frequency and biosecurity measures at farm-level, intended to decrease the SE prevalence in laying flocks and ultimately the number of human SE infections.